Molded records such as conventional audio records or the more recently developed capacitive electronic discs, are conventionally manufactured by compression molding of a thermoplastic composition. The thermoplastic compositions used to mold records are comprised of a thermoplastic resin; such as polyvinyl chloride, polyvinyl acetate, polystyrene and the like, and additives such as stablizers, lubricants, antistatic agents and carbon black. In the compression molding process a preform is initially formed which contains an amount of the thermoplastic composition which is somewhat in excess of the amount of material actually needed to mold the record in order to insure that when the record is pressed it will not have any voids and will be of uniform density.
The preform is placed in a record molding press between a pair of stampers which are heated to an elevated temperature above the melting point of the thermoplastic composition. The molding press in then closed which causes the heated stampers to both heat the thermoplastic composition of the preform above its melting point and to cause the resulting molten thermoplastic composition to flow between the stampers to form a record. During the molding process the excess amount of the thermoplastic composition from the preform will flow past the outer edges of the stampers and will form an irregular ring of material around the outer diameter of the molded record which is referred to as flash. After the molten thermoplastic material has been pressed between the heated stampers, the stampers are then cooled which in turn causes the thermoplastic composition to solidify.
After the molded record is solidified the flash which is formed on the outer diameter of the record must be removed and the outer diameter of the record trimmed to a specified final diameter. Various types of trimming apparatus which are generally well known in the art can be used to remove the flash. The type of trimmers which are commonly used include, for example, scissors cutters, knife cutters, routers and the like.
A serious production problem, which is referred to as breakout, is encountered with all of the commonly used types of trimmers. Breakout occurs in the trimming step when the flash does not separate cleanly from the outer diameter of the molded record and causes a portion of the molded record is broken out along with the flash; or when the edge of the record is cracked; or other similar defects occur, all of which causes destruction of the molded record. Breakout is also often encountered during the final trimming of the outer diameter of the record.
While the exact cause of the breakout problem is not known for certain it is believed to be due in part to the relative brittle nature of the solidified thermoplastic compositions used in the molding of records. This appears to be true particularly with regard to the thermoplastic compositions used for capactitive electronic discs. Capacitive electronic disc must be electrically conductive. In order to obtain the required electrical conductivity the resin compositions are heavily loaded with conductive carbon black which causes substantial embrittlement of the solidified thermoplastic composition.
It would be highly desirable to have apparatus which would prevent breakout during removal of the flash and the final finishing of the outer diameter of the thermoplastic records.